Umthetho ka-Hess: ukuthola u-ΔH ngokwengeza ukusabela

Kungani umsebenzi wesimo wenza umthetho ka-Hess usebenze

I-enthalpy ngumsebenzi wesimo: inani layo lincike kuphela esimweni samanje sesistimu, hhayi ekutheni isistimu yafika kanjani lapho. Ushintsho u-ΔH phakathi kwesimo sokuqala nesokugcina ngakho luyafana kuyo yonke indlela phakathi kwazo. Ukuphakama kusebenza ngendlela efanayo. Noma ugibela intaba ngomzila ogwegwile noma ngamagoxo amade, ushintsho lwakho lokuphakama luyafana.

Umthetho ka-Hess ulandela ngqo. Uma ukusabela kungabhalwa njengesamba samasinyathelo amabili noma ngaphezulu, u-ΔH waso uyisamba samanani e-ΔH amasinyathelo. Amasinyathelo akudingeki enzeke ngempela; adinga nje ukuba izibalo zamakhemikhali ezivumelekile ama-enthalpy azo aziwayo. Leso sikhala yilokho okwenza umthetho ube wusizo ekusabeleni ongakwazi ukukusebenzisa kahle ku-calorimeter.

Imithetho emithathu yokuphatha izibalo ze-thermochemical

Umthetho ngamunye uvela eqinisweni lokuthi u-ΔH ungumsebenzi wesimo owandayo. Ukuphendula inqubo kubuyisela isistimu esimweni sayo sokuqala, ngakho ushintsho lwe-enthalpy kufanele lulingane futhi luphikisane. Ukuphindaphinda kabili inani lokusabela kuphindaphinda kabili amandla abandakanyekayo.

Ngaphambi kokuhlanganisa noma yini, bhala isibalo esiqondiwe phezulu emsebenzini wakho. Bese uthola, ezibalweni ezinikeziwe, into ngayinye evela kanye kuphela. Indawo yayo nenombolo eyandulelayo zikutshela ukuthi lesi sibalo kufanele siphendulwe yini nokuthi siphindaphindwe ngesiphi isici.

• Phendula isibalo futhi ushintsha uphawu lwe-ΔH yaso.
• Phindaphinda isibalo ngesici futhi uphindaphinda u-ΔH waso ngesici esifanayo. Izingxenye ezifana no-½ ziyavunyelwa.
• Engeza izibalo futhi wengeza amanani azo e-ΔH. Izinhlobo ezivela nhlangothi zombili zesamba ziyakhansela, kodwa kuphela uma izimo zazo zenyama zifana.

Isibonelo esisebenzile: ukuphendula nokwengeza izibalo ezimbili

Thola u-ΔH° wokuguqula i-graphite ibe yidayimane, C(graphite) → C(diamond). Ukuguqula ngqo kudinga ingcindezi enkulu futhi akunakwenziwa ku-calorimeter, kodwa zombili izinhlobo ze-carbon ziyashisa. Sebenzisa (1) C(graphite) + O₂(g) → CO₂(g), ΔH° = −393.51 kJ, no-(2) C(diamond) + O₂(g) → CO₂(g), ΔH° = −395.40 kJ. Inani lokuqala livela kuSithasiselo G se-OpenStax Chemistry 2e; lesibili labalwa etafuleni elifanayo.

I-graphite iyisisabelo ku-(1) nasekuqondiweni, ngakho gcina u-(1). Idayimane liyisisabelo ku-(2) kodwa ngumkhiqizo ekuqondiweni, ngakho phendula u-(2): CO₂(g) → C(diamond) + O₂(g), ΔH° = +395.40 kJ. Ukwengeza zombili, i-O₂ ne-CO₂ zivela kanye nhlangothi zombili futhi ziyakhansela, zishiya ncamashi okuqondiwe. ΔH° = −393.51 kJ + 395.40 kJ = +1.89 kJ.

Chaza impendulo. Uphawu oluhle lusho ukuthi idayimane lilele ngaphezulu kancane ku-enthalpy kune-graphite, okuhambisana nokuthi i-graphite, hhayi idayimane, iyifomu lesikhombisi le-carbon. Usayizi omncane wenombolo uyisikhumbuzo sokuthi ukukhipha amanani amabili amakhulu acishe alingane kukhulisa ukungaqiniseki kwawo.

Isibonelo esisebenzile: izibalo ezintathu nokuhlola okuphambene

Thola u-ΔH° we-hydrogenation C₂H₄(g) + H₂(g) → C₂H₆(g) ngedatha yokushisa. Sebenzisa (1) C₂H₄(g) + 3 O₂(g) → 2 CO₂(g) + 2 H₂O(l), ΔH° = −1411.1 kJ; (2) H₂(g) + ½ O₂(g) → H₂O(l), ΔH° = −285.83 kJ; no-(3) C₂H₆(g) + 7/2 O₂(g) → 2 CO₂(g) + 3 H₂O(l), ΔH° = −1560.5 kJ. La manani okushisa abalwe kudatha yokwakheka yoSithasiselo G ukuze ukwazi ukuwaphinda.

U-C₂H₄ no-H₂ bayizisabelo kokubili ekuqondiweni nasezibalweni (1) no-(2), ngakho zigcine njengoba kubhaliwe. U-C₂H₆ ungumkhiqizo ekuqondiweni kodwa uyisisabelo ku-(3), ngakho phendula u-(3) bese ushintsha uphawu lwawo lube ngu-+1560.5 kJ. Manje bala: i-oxygen ingu-3 + ½ = 7/2 O₂ ngakwesobunxele no-7/2 O₂ ngakwesokudla; i-carbon dioxide ingu-2 nhlangothi zombili; amanzi angu-2 + 1 = 3 ngakwesokudla no-3 ngakwesobunxele. Konke kuyakhansela ngaphandle kokuqondiwe. ΔH° = −1411.1 − 285.83 + 1560.5 = −136.4 kJ, kubikwe endaweni eyodwa yedesimali ngoba amanani ancane kakhulu anelodwa.

Ukuhlola okuphambene ngama-enthalpy okwakheka: u-ΔH°f we-C₂H₆(g) ngu-−84.0 kJ/mol kanti we-C₂H₄(g) ngu-+52.4 kJ/mol, futhi u-H₂(g) ungu-zero, ngakho ΔH° = −84.0 − 52.4 = −136.4 kJ. Izindlela ezimbili ezihlukene zinikeza impendulo efanayo, yilokho kanye umsebenzi wesimo ukuqinisekisayo.

Ama-enthalpy ajwayelekile okwakheka nezimo zesikhombisi

I-enthalpy ejwayelekile yokwakheka, ΔH°f, ushintsho lwe-enthalpy lapho i-mole eyodwa yenhlanganisela yakheka kuzakhi zayo, ngasinye esifomini saso sesikhombisi, ezimweni ezijwayelekile. Ifomu lesikhombisi yifomu elizinzile kakhulu ngokuhlukile okungajwayelekile: i-phosphorus isebenzisa i-P₄(s) emhlophe, uSithasiselo G olubhala ku-zero. I-IUPAC ibeka ingcindezi ejwayelekile ku-1 bar; amatafula amaningi asasebenzisa i-1 atm, futhi umehluko awubalulekile emsebenzini omningi we-enthalpy. Amanani abhalwe etafuleni ngokuvamile anikezwa ku-298.15 K.

Isibalo sokwakheka sihlala senza i-mole eyodwa ncamashi yomkhiqizo, ngakho izinombolo ezandulelayo eziyizingxenye ziyajwayelekile: ½ N₂(g) + 3/2 H₂(g) → NH₃(g), ΔH°f = −45.9 kJ/mol. Isakhi esisesifomini saso sesikhombisi, njenge-O₂(g), H₂(g), i-graphite, noma i-bromine eketshezi, u-ΔH°f ungu-zero ngencazelo. Ezinye izinhlobo zesakhi azingu-zero: uSithasiselo G ubhala idayimane ku-+1.89 kJ/mol uma kuqhathaniswa ne-graphite.

Ukubala u-ΔH°rxn kusuka kudatha yokwakheka

Noma yikuphi ukusabela kungacatshangwa njengamasinyathelo amabili: ukuphula izisabelo zibe yizakhi zazo, bese wakha imikhiqizo kulezo zakhi. Isinyathelo sokuqala siyiphindiseleko lokusabela kokwakheka kwezisabelo kanti sesibili ukusabela kokwakheka kwemikhiqizo. Lokho kunikeza ifomula esebenzayo ΔH°rxn = Σ n ΔH°f(imikhiqizo) − Σ m ΔH°f(izisabelo), lapho u-n no-m bezinombolo ezandulelayo esibalweni esilinganisiwe.

Isibonelo esisebenzile. Thola u-ΔH° we-2 N₂H₄(l) + N₂O₄(g) → 3 N₂(g) + 4 H₂O(g), ukusabela kwe-hydrazine okufana nokusetshenziswa ezinjinini ezithile ze-rocket. Kusuka kuSithasiselo G: N₂H₄(l) +50.63 kJ/mol, N₂O₄(g) +11.1 kJ/mol, H₂O(g) −241.82 kJ/mol, no-N₂(g) 0. Imikhiqizo: 3(0) + 4(−241.82) = −967.28 kJ. Izisabelo: 2(50.63) + 11.1 = 112.36 kJ. ΔH° = −967.28 − 112.36 = −1079.6 kJ, kubikwe endaweni eyodwa yedesimali ngoba u-11.1 unelodwa. Lokho ngu-−539.8 kJ ngemole ngayinye ye-hydrazine.

Isimo samanzi siyabalulekile. Uma isibalo sikhiqiza amanzi oketshezi, usebenzisa u-−285.83 kJ/mol, impendulo ibizoba ngu-−1255.7 kJ. Umehluko ngokuphindwe kane kwe-enthalpy ekhululwa lapho i-mole yomhwamuko wamanzi ifingqeka.

Amaphutha aphula ukubala komthetho ka-Hess

Amaphutha amaningi kulezi zinkinga angamaphutha okubhala, futhi ukubala okubhaliwe kwezinhlobo zonke kubamba cishe wonke. Lapho ukukhansela kungakhiphi ncamashi okuqondiwe, isibalo siphendulwe noma sakalwa kabi.

Ukuhlola kwesibili ukulinganisa uphawu. Ukushisa nokwakheka kwezinhlanganisela ezizinzile eziningi kuyi-exothermic, ngakho impendulo ewuhle yokushisa uphethiloli ikhomba isinyathelo esiphendulwe.

• Ukuphendula isibalo ngaphandle kokushintsha uphawu lwe-ΔH yaso.
• Ukuphindaphinda isibalo ngo-2 noma ½ kodwa ushiye u-ΔH waso ungashintshiwe.
• Ukukhipha ngokulandelana okungalungile: ifomula ithi imikhiqizo ukususa izisabelo.
• Ukukhohlwa izinombolo ezandulelayo lapho kwengezwa ama-enthalpy okwakheka.
• Ukusebenzisa u-ΔH°f wesimo esingalungile, njenge-H₂O(g) lapho isibalo sinakho H₂O(l).
• Ukunikeza isakhi sesifomu saso sesikhombisi u-ΔH°f ongesona u-zero, noma ukunikeza inhlanganisela u-zero.